β-galactosidases hydrolyze non-reducing terminal β-D-galactose in β-D-galactopyranosides such as lactose, or catalyze the transition of β-D-galactopyranoside to galactose. Generally, such enzymes have two characteristics (hydrolytic activity and transglycosylation activity) which are all industrially applicable. The hydrolytic activity hydrolyzes lactose in milk and milk products to prevent lactose intolerance and increase sweetness, and the transglycosylation activity is used for production of galactooligosaccharides which promote the growth of lactic acid bacteria that are human intestinal beneficial microorganisms.
Beta-galactosidases are found in various microorganisms, plants and animals, but beta-galactosidases which are currently used in industrial applications are those isolated from microorganisms. Bata-galactosidases having different transglycosylation specificities were isolated from Bacillus sp., Aspergillus sp., Saccharomyces sp., etc., and various studies on thermostable enzymes have been conducted.
Beta-galactosidases which are currently used for production of galactooligosaccharides in the world are mostly those derived from Bacillus sp. or Aspergillus sp. Particularly, beta-galactosidases derived from Bacillus sp., particularly Bacillus circulans, are most frequently used for commercial purposes due to their activation temperature (50 to 60° C.) and high transglycosylation activity. Particularly, a beta-galactosidase derived from Bacillus circulans ATCC 31382 is commercially available under the trade name of BIOLACTA (Amano) This enzyme is known to be produced by culturing Bacillus circulans in a suitable medium, followed by cell disruption or recovery from the medium.
The characteristics of the beta-galactosidase derived from Bacillus circulans ATCC 31382 were reported in several publications. It was reported that the microorganism Bacillus circulans ATCC 31382 contains three structural isoforms of beta-galactosidases. According to the sizes thereof, the beta-galactosidase proteins are designated 212 kDa beta-galactosidase I, 145 kDa beta-galactosidase II, and 86 kDa beta-galactosidase III. The nucleotide sequences of the three isoform genes were identified by Amano for beta-galactosidase I (WO2010/140435), GenoFocus for beta-galactosidase II (Korean Patent No. 1,121,161), and the Ito group for beta-galactosidase III. It was found that among the three isoforms, beta-galactosidase I and II can be used for synthesis of galactooligosaccharides. Until now, various research groups have reported that only three beta-galactosidase genes are present in Bacillus circulans. 
The present inventors expected that, if beta-galactosidase isoforms are present in Bacillus circulans, a beta-glycosidase gene having the characteristics of another additional beta-galactosidase different from the three beta-galactosidases identified to date can be present Bacillus circulans. Based on this expectation, the present inventors have made extensive efforts to identify a novel beta-galactosidase, and as a result, have found that a novel beta-galactosidase different from the three reported beta-galactosidases is present in Bacillus circulans, and have identified the nucleotide sequence of a gene encoding the novel beta-galactosidase, thereby completing the present invention.